The urchins are very closely allied to the starfishes, especially in structure. They have the same kind of feet, and among the spines is seen a singular handlike organ common to the starfishes. It has three fingers (Fig. 55) and a short stem, and is constantly in motion, its office appearing to be to clean the body. Foreign objects are taken up by this peculiar hand and passed on from one to the other until they are finally dropped off. Here is the same madreporic plate or sieve, and the structure of the Echinus (Fig. 56) is very similar to that of the starfish. The former has a long set of jaws, hence is a biter and nipper, while the starfish is a sucker. The shell of the Echinus is really a beautiful object when divested of the spines and bleached in the sun, appearing as pure white as coral after bleaching. It is made up of about six hundred hard, limy plates arranged in double rows, which contain about thirty-seven hundred pores through which the feet protrude. Despite this marvelous supply of feet, or organs of locomotion, the Echinus is a very slow walker. The spines number four thousand or more, and each one works on the ball-and-socket plan, is hollow, and moves readily in all directions. [Illustration: FIG. 55.--Handlike organ of sea urchins and starfishes.] [Illustration: FIG. 56.--Structure of the Echinus: _a_, mouth; _o_, anus; _c_, stomach; _f_, madreporic plate; _d_, intestine; _p_, ambulacra; _v_, heart; _z_, spines.] The sea urchin is produced from eggs. The young pass through some remarkable changes before they assume the adult form. In one of these changes they appear as free-swimming animals (Fig. 57), and resemble anything but the perfectly developed Echinus. Some of the sea urchins of deep water, as the Hermiaster and others, carry their young in pouches, the spines being folded over them to hold them in place. They rarely move from the crevice on the rock which they select. They can be found in the same place for months together, and have a limited power of wearing out the rock. How the Echinus grows inclosed in so hard a shell might be a puzzle did we not know that the shell is covered with a skin, each plate being literally surrounded by it. This skin secretes lime, taking it from the water and depositing it on the edges of all the plates, so that the animal grows rapidly and symmetrically. The Echini are the scavengers of the ocean, and they aid in maintaining the clearness and purity of the water. In some countries certain kinds are eaten, and one species is valuable for its spines, which are used as slate pencils. [Illustration: FIG. 57.--Young sea urchin.] IX. THE SEA CUCUMBERS [Illustration: FIG. 58.--Sea cucumber, showing its breathing organs.]